1. Field of the Invention
The present invention relates to partial response modulation systems and, more particularly, to a partial response modulation system employing quadrature amplitude modulation.
2. Description of the Prior Art
Data transmissions systems generally are designed to avoid the generation of intersymbol interference. However, some systems are known in which intersymbol interference is postively used, often referred to as controlled intersymbol interference. Namely, the impulse response of this system and thus the transmitted waveform by means of which successive symbols are transmitted is controlled such that there is a constant intersymbol interference in one or more bits. This technique of controlled intersymbol interference reduces the frequency spectrum of the impulse response, or waveform, and thus makes more efficient use of transmission bandwidth. Such a method of transmission is referred to as duobinary, and the implementation of this method typically as a partial response system. A partial response system as above mentioned is described in the papers: "Generalization of A Technique for Binary Data Communication" by E. R. Kretzner; IEEE Transactions on Communication Technology, April 1966, Volume COM-14, No. 1, pp 67-68, and "Data Transmission Using Controlled Intersymbol Interference" by K. H. Schmidt; Electrical Communication Volume 48, No. 1 and 2, 1973.
Partial response systems are classified into a total of five types, designated class 1 to class 5. Each of these types differs in the means of the superimposition of the impulse response; where the input signal is a 2-level signal, the superimposition results in a multi-level signal.
In class 1 partial response systems, an input pulse is converted into a pulse of a doubled width and accordingly preceding and succeeding pulses are superimposed. Thus, a 2-level digital input signal becomes a 3-level signal.
In such a partial response system, it is necessary to accumulate the preceding element (bit) group in order to demodulate the signal which was subjected to partial response conversion, thereby to return to the initial 2-level signal. Because of this, a problem arises in that erroneous propagation occurs during demodulation. Thus, symbol processing, called precoding, is performed before the partial response conversion. To amplify, for a 2-level digital signal having the levels "0" and "1" and converted in accordance with partial response conversion to the levels of "0", "1", and "2", for example, the code conversion is always performed so that the level "1" of the converted, partial response signal corresponds to the level "1" of the initial 2-level input digital signal.
Quadrature amplitude modulation systems as well are known in the prior art. In such systems, quadrature-related carriers, i.e., carriers of the same frequencies but having a phase difference therebetween of 90.degree., are independently amplitude modulated and then combined into a QAM (Quadrature Amplitude Modulation) signal. An ordinal 4 phase PSK (Phase Shift Keying) modulation system is a kind of QAM system. The configuration of the quadrature amplitude modulation system is shown, for example, in FIG. 8 of the U.S. Pat. No. 3,806,807.
Demodulation of a QAM signal at the receiver, or receiving side of this system, is performed by regenerating the carrier and synchronously detecting the received QAM signal with the regenerated carrier.
In this operation, if the phase of the regenerated carrier is not determined and/or varies, these result errors in the demodulated data. The circumstance that the phase of the regenerated carrier is not determined properly is generally referred to as the phase ambiguity of the regenerated carrier.
In ordinal 4-phase PSK systems, differential phase modulation techniques and systems are known which assure correct demodulation in spite of phase ambiguity of the regenerated carrier; in such demodulation, code processings are performed by the differential logic circuit. However, in partial response modulation systems which perform quadrature amplitude modulation by the digital signal (a 3-level signal) which has been subjected to partial response conversion in accordance with techniques related to the present invention (and as later described herein, the transmitted QAM signal has a total of nine phase conditions), techniques for insuring proper demodulation in spite of phase ambiguity of the regenerated carrier at the receiver are currently not known.